Combine flour with baking powder, baking soda, and salt - set aside.
Whip together the eggs and oil.
Add the sugar, followed by the sour cream.
Next, add the vanilla.
Lastly, add the flour mixture. Combine well.
Pour into the prepared baking pan. Use an offset spatula to spread evenly.
Generously top with the prepared streusel mixture.
Bake for 35 to 40 minutes or until a skewer inserted in the center comes out clean.
Let cool in the pan for at least 10 minutes before slicing.
Tips for making this coffee cake
Crumble the streusel until you have close to breadcrumb consistency. Leave some big size crumbs as the look and taste wonderful.
You can add 1/4 tsp cinnamon to the streusel mixture
You can replace the white sugar to brown sugar in both the streusel as well as the cake.
Whip the eggs with oil well. This is what gives us a light and airy cake.
Whip the sour cream with a whisk before adding it to the batter to prevent lumps
Use good quality vanilla extract as this is a vanilla based cake batter.
You can add 1/2 tsp cinnamon, 1/4 tsp ground ginger, and 1/8 tsp ground nutmeg to this batter.
If you don't have a 9 x 9-inch square pan you can also use an 8-inch round cake pan.
Coffee cakes taste best in the first two days. This cake can be kept at room temperature for 3 to 4 days. Wrap the cake well to prevent drying out.
This cake can also be frozen in the freezer for up to a month. Personally, I like to cut the cake into slices and place the slices in a freezer-safe storage bag. Then take only as many slices as I need for my tea time snack.